Steam cracking, also referred to as pyrolysis, has long been used to crack various hydrocarbon feeds into olefins. Light olefins such as ethylene, propylene, and butene are preferred. Conventional steam cracking utilizes a pyrolysis furnace wherein the feed is heated sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis furnace has two main sections: a convection section and a radiant section. The hydrocarbon feed enters the convection section of the furnace either already as a vapor or as a liquid which is vaporized through direct contact with steam and indirect contact with the burners in the radiant section. The vaporized feed then proceeds to the radiant section where cracking takes place. Non-volatiles in the convection section cause coking, and coking often results in costly shutdowns for cleaning.
Lower cost heavy feedstocks often include non-volatile components. These components are often tars and asphaltenes (collectively “asphaltenes”) which are molecules of high molecular weight with multi-ring structures. Asphaltenes can be present in hydrocarbon feeds as naturally-occurring components, contaminants (e.g., those introduced during transport) and/or by-products formed during feed processing. In addition to causing coking, asphaltenes reduce the economic value of the hydrocarbon feed by rendering it less compatible for mixing with highly paraffinic streams, further inducing precipitation of the asphaltenes from the resulting mixture.
Various methods are known in the art to treat various feedstocks to reduce the asphaltenes content.
U.S. Pat. No. 3,948,756 discloses pentane insoluble asphaltene removal from residual petroleum oil by pretreating the feeds with hydrogen, resulting in a maximum conversion of asphaltenes when 5 to 45 volume percent of the 975° F.+ fraction in the feedstock is converted to lower boiling fractions.
U.S. Pat. No. 4,719,007 discloses the removal of trace asphaltenes from a hydrotreated feed by contacting with a carbon adsorbent.
U.S. Pat. No. 5,107,060 discloses treatment of mercury-containing hydrocarbon feedstocks such as liquid condensate from natural gas fields for use as a steam cracker feed by contacting with a mercury reactive adsorbent and a high surface area support such as active carbon.
U.S. Pat. No. 5,374,350 discloses hydrotreating hydrocarbon oil feedstock in the presence of a catalyst composition comprising (a) an activated carbon component derived from lignite possessing a pore volume in the 100 to 400 angstrom pore diameter range of at least about 0.08 cc/g., and an average pore diameter of 15 to 100 angstroms, and (b) a molybdenum or tungsten component and (c) a cobalt or nickel component.
In U.S. Pat. No. 6,245,223 a major portion of metals and coke precursors is removed from a hydrocarbon stream which may be effluent from a solvent deasphalter which includes a pentane solvent, by contacting with a hydrocarbon insoluble adsorbent selected from silica, silica-alumina, acid treated clays and activated carbons, recovering the oil which does not adsorb and removing the metals and coke precursors from the adsorbent.
In U.S. Pat. No. 6,303,842 pentane insolubles are removed from a residuum used as a steam cracking feed by solvent extraction prior to use.
“Effect of Clay Particle Size on the Adsorption of a Pentane Insoluble Bitumen Fraction,” Tu Yun et al., Clay Sci, pp. 194-198 (2006), discloses the effect of clay particle size on the adsorption of a well-characterized pentane insoluble fraction (PI) from Athabasca oil sands bitumen by clay minerals.
U.S. patent application Ser. No. 11/958,433, filed Dec. 18, 2007 discloses thermally cracking hydrocarbon feeds containing paraffin insoluble compounds by selectively adsorbing paraffin-insoluble compounds with solid particulate adsorbents, prior to cracking. An aromatics-containing stream such as one derived from cracked product is used to desorb the paraffin insoluble compounds from the adsorber stage.
It would be desirable to upgrade asphaltene-containing hydrocarbon streams to provide a suitable stream for cracking using a regenerable adsorbent exhibiting a long adsorbent life. The present inventors have discovered a process for removing asphaltene contaminants in hydrocarbon streams by a carbon based adsorbent (preferably regenerable) under conditions which provide enhanced adsorbent life.